Exopolysaccharides (EPSs) are structurally and functionally valuable biopolymer secreted by different prokaryotic and eukaryotic microorganisms in response to biotic/abiotic stresses and to survive in extreme environments. Microbial EPSs are fascinating in various industrial sectors due to their excellent material properties and less toxic, highly biodegradable, and biocompatible nature. Recently, microbial EPSs have been used as a potential template for the rapid synthesis of metallic nanoparticles and EPS-mediated metal reduction processes are emerging as simple, harmless, and environmentally benign green chemistry approaches. EPS-mediated synthesis of metal nanoparticles is a distinctive metabolism-independent bio-reduction process due to the formation of interfaces between metal cations and the polyanionic functional groups (i.e. hydroxyl, carboxyl and amino groups) of the EPS. In addition, the range of physicochemical features which facilitates the EPS as an efficient stabilizing or capping agents to protect the primary structure of the metal nanoparticles with an encapsulation film in order to separate the nanoparticle core from the mixture of composites. The EPS-capping also enables the further modification of metal nanoparticles with expected material properties for multifarious applications. The present review discusses the microbial EPS-mediated green synthesis/stabilization of metal nanoparticles, possible mechanisms involved in EPS-mediated metal reduction, and application prospects of EPS-based metal nanoparticles.

In the present study, a marine sponge-associated endosymbiotic bacterium Bacillus megaterium MSBN04 was evaluated for exopolysaccharide (EPS) production. The production process was optimized by central composite design (CCD). The productivity was increased up to 5.62 g L⁻¹ with sucrose as sole carbon source. The secreted EPS was characterized by NMR analysis, confirming the presence of monosaccharide units such as α-D-glucose and α-D-galactose, which further confirms that the secreted EPS is a heteropolysaccharide. The purified EPS showed considerable flocculating activity (45.41%) with 4 mg L⁻¹ of EPS. Using EPS as reducing and stabilizing agent, gold nanoparticles (AuNPs) were synthesized. The synthesized AuNPs (5–20 nm) were of spherical crystalline nature and capped with an EPS layer and were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The synthesis of AuNPs was dependent on the concentration of EPS. The synthesized AuNPs showed significant antibacterial activity against clinical pathogenic bacteria. Hence, EPS-mediated synthesis of AuNPs is an alternative approach to chemical synthesis and thus it is an environmentally benign, greener and economical approach.